1. Technical Field
The present invention relates to an imaging apparatus including a solid-state imaging device having a large number of effective pixel areas that receives subject light to generate charges and a large number of OB pixel areas which are light shielded and output black level determination signals, and also relates to a photographing control method.
2. Description of the Related Art
FIG. 12 is a plan view showing the schematic configuration of a solid-state imaging device of a CCD (Charge Coupled Device) type according to a related art.
A semiconductor substrate made of silicon or the like is formed with a pixel area 100 having an effective pixel portion 200, an OB (optical black) portion 300 and an OB portion 400.
In the pixel area 100, a large number of lines in each of which a large number of photoelectric conversion elements such as photodiodes are arranged in a horizontal direction are arranged in a vertical direction which is perpendicular to the horizontal direction.
A charge generated by each photoelectric conversion element in the pixel area 100 is read out to a vertical charge transfer path (not shown in FIG. 12) in the pixel area 100, and transferred in the vertical direction. Charges of one line transferred in the vertical charge transfer path are transferred in the horizontal direction in a horizontal charge transfer path 500. At the end of the horizontal charge transfer path 500, an output section 600 such as a floating diffusion amplifier (FDA) converts the charges into voltage signals (hereinafter may be referred to as “imaging signals”) according to the charge amounts, for output. Thereby, the charges transferred in the horizontal direction are converted into the voltage signals in this output section 600 and output to an outside.
The OB portion 400 is an area formed of several lines, which are completely light shielded, at the end of the pixel area 100 on the horizontal-charge-transfer-path side. The respective photoelectric conversion elements formed in the OB portion 400 constitute an OB pixel area that outputs black level determination signals. In the following description, it is assumed that the number of photoelectric conversion elements in the OB portion 400 is 1200.
The OB portion 300 is an area formed of several (10 in the following explanation) photoelectric conversion elements, which are completely light shielded, in each line at the opposite end to the output section 600, excluding the several lines in the OB portion 400. The respective photoelectric conversion elements formed in the OB portion 300 also constitute the OB pixel area that outputs the black level determination signals.
The effective pixel portion 200 is an area in which the photoelectric conversion elements other than the photoelectric conversion elements in the OB portion 300 and the OB portion 400 are formed. Openings are formed in a light shading film that is provided above the respective photoelectric conversion elements of the effective pixel portion 200, and subject light can enter through the openings. The respective photoelectric conversion elements formed in the effective pixel portion 200 constitute an effective pixel area that receives the subject light to generate charges. In the following description, imaging signals output from the effective pixel area may also be called “effective signals”, and imaging signals output from the OB pixel area may also be called “OB signals”.
An imaging apparatus having the solid-state imaging device includes a CDS circuit, an amplifier, an AD converter and a clamp circuit. The CDS circuit performs a correlation double sampling process for each imaging signal output from the solid-state imaging device to extract a data component excluding a noise component from each imaging signal for output. The amplifier amplifies the imaging signal output from the CDS circuit. The AD converter converts the imaging signal output from the amplifier into a digital signal. The clamp circuit calculates a moving average of n (n is a natural number of 2 or greater; in the following explanation, it is assumed that n=100) OB signals among the imaging signals output from the AD converter and determines a black level of the effective signals.
FIG. 13 is a view for explaining an operation of the solid-state imaging as shown in FIG. 12 at the time of a normal photographing.
After the end of exposure, unwanted charges in the vertical charge transfer path are swept out at high speed, and an empty transfer is performed. Then, a charge generated in each photoelectric conversion element during an exposure period is read out to the vertical charge transfer path as shown in FIG. 13. For a period from the start of exposure to the end of reading, no signal is input into the clamp circuit. Therefore, a black level which is determined based on the clamp circuit is a constant low value.
After the end of reading the charges, the charges in each line are sequentially transferred onto the horizontal charge transfer path 500, then transferred to the output section 600 and output as imaging signals during a signal output period. During this signal output period, first of all, the OB signals are output successively from the photoelectric conversion elements in the OB portion 400, and data components are extracted from the output OB signals, amplified, converted into the digital form, and input into the clamp circuit.
Assuming that an i-th OB signal input into the clamp circuit is OB(i), the clamp circuit calculates a black level by obtaining an integral value of the input OB signals and dividing this integral value by n (=1,000). For example, if OB(1) is input, OB(1)/1000 is calculated, and if OB(2) is input at the next time, {OB(1)+OB(2)}/1,000 is calculated. In this manner, the black level is updated every time the OB signal is input. In the case where the OB signal following OB(1,001) is input, the oldest OB signal is subtracted from the integral value to be divided by 1,000, the latest OB signal is added to the integral value, and the integral value is divided by 1,000 to update the black level.
Since the clamp circuit performs such an arithmetical operation process, the black level determined by the clamp circuit gradually increases as the number of input OB signals increases, as shown in FIG. 13. And, at the time when 1,000 OB signals are input, the calculated black level is almost stabilized, and after that, the black level transits without great variation.
After 1,200 OB signals are output from the OB portion 400, an operation of (i) outputting the effective signals from each line in the effective pixel portion 200 and the OB portion 300, and (ii) then outputting the OB signals from the same line is performed for all the remaining lines.
JP 2002-300478 A (corresponding to US 2002/0140844 A) describes a method for determining a black level based on imaging signals output by the empty transfer from a solid-state imaging device as shown in FIG. 13. However, since the imaging signals obtained through the empty transfer are different from those obtained in a state where the photoelectric conversion elements are light-shielded, it is difficult to determine the black level precisely.
Minimizing the solid-state imaging device has significantly increased the number of pixels (=photoelectric conversion elements) in recent years. Examples of a method for effectively using the increased number of pixels include a high-sensitivity photography by a pixel mixing drive (a driving method for mixing charges read from photoelectric conversion elements so that a total number of resultant charges is 1/M of the read-out charges and outputting imaging signals according to the charges after mixing). The pixel mixing drive has a merit of improved continuous shooting speed because of the decreased number of imaging signals, as well as an improved S/N due to an increased output signal amount.
However, the inventor found that if the pixel mixing drive is performed in the imaging apparatus shown in FIG. 12, the number of OB signals output from the OB portion 400 decreases to 1/M, so that a black level cannot be stabilized before the start of outputting the effective signals from the effective pixel portion 200, which degrades an image quality.
FIG. 14 is a view showing an operation sequence of the solid-state imaging device shown in FIG. 12 at the time of pixel mixing drive.
The operation from the start of exposure to the reading of charges is the same as that in FIG. 13. After reading the charges, the charges are mixed, for example, in the vertical charge transfer path so as to cause the total number of charges read from the photoelectric conversion elements to be half, and the charges after mixing are transferred. In this case, only the 600 OB signals are output from the OB portion 400. Therefore, at the time when the output of the OB signals from the OB portion 400 is completed, the black level does not reach a stable level as shown in FIG. 14.
To stabilize the black level, it is required that the remaining 400 OB signals are input into the clamp circuit. Since there are only 10 OB pixel areas per line in the OB portion 300, it is required that the imaging signals for 40 lines are output from the solid-state imaging device during a period from the start of outputting the effective signals to the completion of outputting the 400 OB signals. For a period of outputting the imaging signals for 40 lines, the black level determined by the clamp circuit rises toward the stable level. Therefore, the levels of the effective signals, which are obtained from the respective lines with reference to the black levels, would vary line by line during this period.
Accordingly, when shooting is performed in a dark place, for example, an image generated based on the effective signals output from the solid-state imaging devices would be colored in part corresponding to the first 40 lines as shown in FIG. 15. In this way, since the black level is not stabilized on the entire screen, a wrong black level (“floating black” or loss of detail in the shadows) is generated, which degrades an image quality. At the time of pixel mixing drive, the black level of the effective signals is so large that such degradation in the image quality is more conspicuous.